In a quantum computer there are two basic types of errors that can occur, bit-flips and phase-flips. Conventional quantum error codes, like the surface code, need to be designed to correct both types of errors. In a cat qubit based architecture, the bit flips are greatly reduced by the hardware architecture that used so that the error correction codes only need to correct for phase flips. A key advantage of this approach is that it could significantly reduce the number of physical qubits needed to produce a logical qubit.
In the experiment that AWS just published in a preprint posted on arXiv, they fabricated a device that contained five data qubits and 4 ancilla qubits and implemented both distance 3 and distance 5 repetition codes to show error correction. These codes were able to operate below the threshold which means that the logical qubit that these encode provides an improvement over the physical error rate. Specifically, this they were able to demonstrate an average error rate of 1.75(2)% for the distance 3 and a slightly improved error rate of 1.65%(3) for the larger distance 5 code. So they did achieve better error rates with the larger code, although just barely. They were also able to demonstrate repeated error correction cycles with a duration of 2.8 µs. per cycle. But still, these error rates are still many orders of magnitude larger than what we would like to see for a full fault tolerant quantum computer (FTQC).
This certainly is another good step forward, but there is much more work to be done. GQI believes that the three challenges which need to be met for a full FTQC for creating a useful quantum computer are the following:
- Challenge 1: Platform capable of scaling to commercially relevant size
- Challenge 2: Code scaling for systematically suppressing logical errors
- Challenge 3: Universal fault-tolerant circuits with realistic clock times
For more about this research, you can access the paper titled Hardware-efficient quantum error correction using concatenated bosonic qubits on arXiv as well as some comments about it posted here on X by Professor John Preskill.
September 28, 2024
